What is the difference between dofollow backlink and nofollow backlink?
 

Actually, my professor said that when I build backlink, I must consider this backlink is dofollow or nofollow.
How can I differentiate dofollow and nofollow backlinks? is there any tool help me do this?
Thank you.

Search engine crawlers crawls the whole content of the web page at that time it finds a link,if the link is do follow link than crawler follow the links and links to another webpage. No follow links are ignored by the search engine crawlers.